1. Field of the Invention
The present invention relates to a waveguide band reject filter employing TEM coaxial type resonators that partially protrude into the top wall of the waveguide in such a way as to produce a predetermined frequency selective discontinuity. By proper choice of location, number of resonators, resonator configuration and protrusion, a spurious free highly efficient frequency selective band reject filter response can be obtained.
2. Description of the Prior Art
Microwave filters are used to provide frequency selectivity, that is, to pass certain frequencies and reject others by means of a group of reactive circuit elements. The prior art of specific interest in this case involves band reject filters that are designed to eliminate a specific frequency band within a much larger spectrum.
Band reject filters may be designed using various techniques that employ, for example, lumped elements with discrete coils and capacitors, stripline, coaxial resonators or waveguide resonators. The present invention to be described operates in a waveguide system, so the discussion of prior art will be confined to waveguide band reject filters.
A waveguide band reject filter consists of a section of waveguide having a direct path from input to output. Individual waveguide resonant cavities are then mounted on the waveguide section and connected to it through iris openings in the waveguide section. FIGS. 1A and 1B illustrate a three resonator waveguide band reject filter where the waveguide resonant cavities are mounted on the broad wall of the main waveguide and coupled to it through iris slots to produce the proper frequency selective discontinuity. FIG. 2 presents the response of this filter showing a rejection notch of greater than 50 dB.
A similar response can also be obtained when double ridge waveguide is used between the input to output ports. The coupling from the rejection resonator to the main guide is obtained by moving the iris coupling slot off center so that it is adjacent to the ridge, i.e., between the ridge and the side wall. The reject resonators are still mounted as shown in FIGS. 1A and B.
Both the standard waveguide and ridge waveguide filters described are very efficient and serve a purpose. However, the resonant waveguide cavities, common to both designs, have a second spurious response that will occur at less than twice the fundamental resonant frequency as shown in FIG. 2. This is explained and expected because the fundamental resonance occurs at a frequency where the waveguide wavelength is 1/2 wavelength long and the next resonance occurs at a frequency where the waveguide wavelength is one wavelength long.
The wavelength in the waveguide is not a linear function since it approaches infinity when the resonant frequency approaches the cut-off frequency of the waveguide. Thus, the relationship between the first and second resonance in the waveguide can be, say 1.7 to 1 rather the 2.0 to 1. In any case, a second rejection response will occur at somewhat less than twice the desired first response frequency.
In many cases where a reject filter is needed, the added spurious response is of no significance. This is especially true when the spurious rejection falls outside the range of the waveguide. However, where a broad frequency spectrum is required, such as that obtained with ridge waveguide, the spurious rejection notch can fall within the desired pass band. For example, the pass band of a particular ridge waveguide may be 6.5 GHz to 18.0 GHz. Thus, with a notch frequency set at 9.0 GHz, the undesired spurious notch will occur at 15.5 GHz and produce undesired attenuation at that frequency.
The object of this invention is to provide a waveguide band reject filter that has no spurious response occurring within the waveguide frequency range.